It is known that among various steel materials, austenitic stainless steel is susceptible to carburization. For example, a cracking tube, which is used for the thermal decomposition reaction in an ethylene manufacturing process of a petrochemical plant, is made of austenitic stainless steel, and carburization occurs on its inner surface after being used for long hours. Moreover, in the manufacturing process of the cracking tube, carburization occurs when heat treatment is performed in a poorly degreased condition of lubricant. Since the occurrence of such carburization may cause significant reduction of the life of the cracking tube, there is a need for accurately sensing whether carburization occurs or not.
For this reason, conventionally, an electromagnetic test such as an electromagnetic induction test is carried out on a cracking tube installed in a plant as a nondestructive inspection across the entire length of the cracking tube at the time of periodic maintenance of the plant so that whether carburization occurs or not is sensed based on the magnitude of the output value thereof. Moreover, also in the manufacturing process of a cracking tube, whether carburization occurs or not is sensed by performing an electromagnetic test across its entire length or by cutting off both ends thereof and performing a microstructure observation thereon.
In general, if a seamless tube is subjected to drawing-working in a producing process, the tube has smaller roughness in the inner surface, so that amount of lubricant adhering on the inner surface becomes smaller. Hence, heat treatment in a poorly degreased condition causes microscopic carburization. Particularly, if the drawing-working is performed in a high-pressure container, the inner surface of the tube becomes almost equal to a mirror surface, and thus carburization due to the poor degrease becomes extremely microscopic.
There have been suggested various methods of sensing whether carburization occurs or not including those that have not been put to practical use (see Patent Literature 1 to Patent Literature 7, for example), but none of these methods can sense the aforementioned microscopic carburization.